To regulate a time base automatically, particularly of a timepiece, such as a watch, a VLF multi-frequency receiver is generally clocked on the basis of a timepiece quartz. The same is true for adjusting the resonant frequency of the receiver antenna, which must be able to pick up radio-synchronous signals. The antenna may be formed of a ferrite core around which a metal wire is wound to allow it to pick up such radio-synchronous signals.
A standard radio-synchronous signal receiver, which is incorporated into a watch, is usually a direct receiver for picking up signals, which may have a frequency of close to 77 kHz. The advantages of this type of receiver are simplicity and low power consumption. However, the frequency of the radio-synchronous signals to be picked up may be different from the aforementioned frequency. Consequently, for every radio-synchronous signal frequency that it wants to receive, the receiver has to have an individual filter, in the form of a specific quartz, outside the main integrated circuit of the receiver. This constitutes a drawback of this standard type of receiver.
US Patent Application No. 2009/0185615 discloses a radio-synchronous receiver, which includes a time code for correcting the time base of a watch. This receiver includes a receiver unit for picking up said radio-synchronous signals, which are frequency converted in a mixer by oscillating signals provided by an oscillator circuit. The intermediate signals supplied by the mixer are filtered by at least one bandpass filter. The filtered and amplified intermediate signals are supplied to the time code detection circuit to supply time data to a central processing unit, which decodes the time from the time data to correct the time base. A radio-synchronous signal reception channel is selected in the receiver so as to configure the oscillator circuit. The configured oscillator circuit supplies oscillating signals whose frequency depends on the selected channel frequency of the radio-synchronous signals to be picked up. However, the oscillating signal frequency is not automatically adapted in accordance with the incoming signal frequency so that the frequency of the intermediate signals is within the frequency band of the bandpass filter.
Like the preceding document, U.S. Patent Application No. 2006/023572 discloses a radio-synchronous signal receiver for correcting the time base of a watch. At the receiver input, a frequency selection circuit is controlled by a processing unit to adapt to the frequency of the incoming radio-synchronous signals, which may have a frequency of 40 kHz, 50 kHz or 60 kHz. The incoming signals are frequency converted in a mixer by oscillating signals supplied by a quartz oscillator. With the quartz oscillator frequency set at 50 kHz, it is possible to pick up radio waves at 40 kHz or 60 kHz with intermediate signals at the mixer output at a frequency of around 10 kHz. A bandpass filter at the mixer output can be centred on 10 kHz to filter the intermediate signals. The filtered and amplified signals are then supplied to a detection circuit connected to a demodulator for correcting the time base. If the value of the radio-synchronous signal frequency is the same as the oscillating signal frequency, the processing unit momentarily disconnects the oscillator circuit. However, the oscillating signal frequency is not adapted to adjust automatically the intermediate signal frequency in the frequency band of the bandpass filter in accordance with the incoming radio-synchronous signal frequency.
U.S. Pat. No. 6,704,554 discloses an FM (frequency modulation) receiver, which can be used for receiving RDS signals. This receiver includes an antenna for picking up signals within the FM transmission band between 88 and 108 MHz. The data signal frequency in the incoming signals is around 57 kHz (sub-carrier) for RDS data or 38 kHz for audio data. These data signals cannot, however, correct the time of a time base. A mixer is also provided for frequency converting the signals shaped by an RF input stage by means of oscillating signals supplied by a local oscillator. Intermediate signals at a frequency of around 70 kHz are supplied at the mixer output and are filtered in a bandpass filter and amplified prior to being supplied to a demodulator. An automatic frequency controller is also provided for adapting the frequency of the oscillating signals from the local oscillator to guarantee a constant frequency for the intermediate signals at the mixer output. However, this complex receiver cannot correct the time of a watch time base. Moreover, this receiver is not provided for the purpose of automatically adjusting the frequency of intermediate signals within the bandpass filter frequency band in accordance with the incoming radio-synchronous signal frequency.
The antenna frequency of this standard receiver must also be tuned to the receiving frequency. This operation is performed by external capacitors, which are normally selected during manufacturing steps in accordance with the frequency of the radio-synchronous signals likely to be picked up. These external capacitors also tune the receiving frequency when the receiver is switched on with compensation for tolerances and the capacitors can be switched depending upon the application for which the receiver can be used. All of these adaptation steps with external components are long and expensive, which constitutes another drawback of this standard type of receiver.
Another state of the art receiver that can be cited concerns EP Patent Application No. 1 666 995 A2, which discloses a watch fitted with a radio-synchronous signal receiver for setting the time of the watch. To achieve this, the receiver includes, in particular, an antenna, means for adapting the receiving frequency in conjunction with the antenna, means for receiving signals picked up by the antenna, processing means connected to a memory, which receive a time code signal from the receiving means for setting the time.
The resonant frequency adapting means for receiving radio-synchronous signals is mainly formed of an array of variable capacitance diodes. These variable capacitance diodes can be selectively placed in parallel to the coil-shaped antenna via a control signal supplied by the processing means. The control signal is a function of a capacitive value stored in the memory to select the number of diodes to be placed in parallel to the antenna in accordance with the frequency of the radio-synchronous signals to be picked up. Only a certain number of capacitive values are stored for adapting the antenna reception frequency. This constitutes a drawback, since the resonant frequency of the antenna is not precisely defined for receiving radio-synchronous signals at a determined frequency, in the best possible conditions.
EP Patent Application Nos. 1 630 960 and 1 698 950 also disclose an array of switchable capacitors that can be placed in parallel to the antenna for receiving radio-synchronous signals for adapting the resonant frequency of the antenna. The antenna resonant frequency is thus adapted on the basis of the known frequency of the incoming radio-synchronous signals. The radio-synchronous signals thereby picked up supply time data for correcting the time base of a watch. However, the resonant frequency is not automatically adapted after an incoming radio-synchronous signal frequency measurement to allow proper demodulation of the time data to be carried out.
The receiving means include a variable gain amplifier for amplifying the radio-synchronous signals, a filter for filtering the amplified signals and a detection circuit receiving the filtered signals to supply a time code signal to the processing means. The filter includes several quartz crystals, which can be individually selected in accordance with the frequency of the incoming radio-synchronous signals. The detection circuit also controls the amplifier gain. One drawback of the receiving means is that it has to be fitted with several quartz crystals for the filter so that proper filtering can be performed in accordance with the incoming radio-synchronous signal frequency, which makes the receiver expensive.
We can also cite WO Patent Application No. 2006/054576, which discloses a VHF radio signal receiver. This receiver is arranged in a very flexible manner for assembly with various receiving antennas. To achieve this, two switches, controlled by a control logic circuit, are provided at the input for connecting one or other of the receiving antennas. An array of switchable capacitors is also placed in parallel to one or other of the antennas to be used for adapting the resonant frequency of the selected antenna. One drawback of this receiver is that it uses several antennas for receiving radio-synchronous signals. Another drawback is that the resonant frequency of the selected antenna is adapted on the basis of a stored capacitive value, which means that the resonant frequency cannot be automatically adapted in accordance with the incoming signal frequency.
DE Patent No. 35 40 380 discloses a superheterodyne receiver circuit. A switch is provided at input for switching two ferrite core antennas. The input stage also includes, after the antenna, an amplifier, a quartz filter at 77.5 kHz, a mixer for mixing the signals picked up by the antenna with the signals, supplied by a quartz oscillator, which are at a frequency of around 77.283 kHz. A bandpass filter is provided at the mixer output, followed by a shaping unit for supplying time correction signals to a microcontroller connected to a timepiece quartz (32.768 kHz). One drawback of this receiver circuit is that it also includes several selectable antennas at the input thereof. Moreover, there is nothing provided for adapting the receiver circuit in accordance with the incoming radio-synchronous signal frequency.